Stellar parallax

Distances to stars comparatively near to our Sun can be measured using a technique known as trigonometric parallax. This can only be done if the star is close enough to show an apparent shift in position relative to much more distant background stars, shown in the upper right of this image. This requires viewing the star on two different occasions, when Earth is on opposite sides of the Sun (seen lower left). Telescopes like Hubble can then precisely measure the very small angular displacement of the star between observations. When this value is combined using geometry with the value for Earth's orbital diameter, a precise distance can be calculated. Land surveyors commonly use this triangulation technique.